Rocks in your Gas Tank

Experiments onboard the International Space Station
could accelerate the drive toward a hydrogen-based economy.

April 17, 2003: Imagine pulling
up to a filling station, inserting the nozzle into the tank and
the gas flowing into your tank is ... hydrogen. It's colorless,
odorless and the byproduct of burning hydrogen is water vapor,
quickly and safely absorbed by the environment. One pound of
hydrogen supplies three times as much energy as a pound of gasoline.
And it's the most plentiful element in the universe! No wonder
scientists are trying to figure out how to make hydrogen work
as a practical fuel.

"Dozens of companies, including all the major automobile
manufacturers, have designed engines that burn hydrogen--they're
a lot like the internal combustion engines we have in cars today,"
says Al Sacco, director of the NASA-supported Center for Advanced
Microgravity Materials Processing (CAMMP) at Northeastern University
in Boston. "Fuel cells--another possible source of power
for cars--use hydrogen, too. To make these technologies work
in the real world, scientists must find a way to store and transport
hydrogen safely at a cost comparable to that of gasoline."

It's not easy: Hydrogen gas is light and elusive. Tiny
H2 molecules like to sneak through cracks and seals--and
once free they quickly disperse. Hydrogen diffuses four times
faster than methane and ten times faster than gasoline vapors.
This is great for safety because a leak is quickly diluted and
rendered harmless. It's a headache for anyone who wants to store
the gas.

Liquid hydrogen is more compact and easier to contain, but
it can be troublesome, too. Hydrogen liquefies at a temperature
of about 20oK (-253oC). Maintaining a tank
full of liquefied hydrogen requires a heavy cryogenic support
system, which may not be practical for passenger cars. Liquid
hydrogen is actually cold enough to freeze air. This could cause
plugged valves and unwanted pressure build-ups. Insulation to
prevent such problems adds to the weight of the storage system.

How can we overcome these obstacles? Simple: put rocks in
your gas tank.

Not
ordinary rocks. Zeolites. Sacco explains: "Zeolites are
porous, rocky substances that act like molecular sponges. In
their crystalline form, zeolites are threaded by a network of
interconnected tunnels and cages, similar to a honeycomb."
A fuel tank lined with such crystals might be able to trap and
store hydrogen gas "in a liquid-like state--without heavy
cryogenics." With support from NASA's Space Product Development
program at the Marshall Space Flight Center, Sacco and colleagues
at CAMMP are working to make zeolite gas tanks a reality.

Left: Zeolite crystals form in a number of complex
shapes that make them highly absorbent.

The name zeolite comes from the
Greek words "zeo" (to boil) and "lithos"
(stone), literally meaning "the rock that boils." This
is because zeolites give up their contents when heated.

Sacco described how a temperature-controlled
zeolite gas tank might work: "We would add some negatively-charged
ions to the zeolite. These ions act like caps, just like caps
on an ink bottle; they block the zeolite's crystalline pores.
By heating the tank--just a little--we can make the ions move
away from the pores. We fill the zeolite with hydrogen, drop
the temperature back to normal, and the ions slide back in place,
sealing off the exits."

Nearly 50 kinds of zeolites with different chemical compositions
and crystal-structures are found in nature, and chemists have
figured out how to synthesize many more. Anyone with a cat has
seen some: they act as odor-absorbers in kitty litter. "The
zeolites we have now can store quite a bit of hydrogen,"
notes Sacco. "But not enough."

How much is enough?

Picture this: Your car's fuel tank is lined with crystallized,
porous rock and that "rock" weighs 93 pounds. You pull
into a hydrogen fueling station and the attendant forces 7 pounds
of hydrogen into the zeolite-lined walls of the tank. This, theoretically,
would be the hydrogen equivalent to a full tank of gasoline--in
both total weight and energy content.

Right: The gas tank of a Chevy Camaro. Automakers would
like hydrogen fuel tanks to be about the same size and weight--and
hold the same amount of energy. Image credit and copyright: CamaroMuscle.com.

"If we can grow zeolite crystals that hold 6% to 7% of
their own weight in hydrogen," says Sacco, "then a
zeolite tankful of hydrogen would be competitive with an ordinary
tankful of gasoline." The best existing zeolites can hold
only 2% to 3%, however.

In 1995, Sacco traveled to space as a mission specialist onboard
the space shuttle Columbia (STS-73). His purpose: to grow better
zeolite crystals. "In low-gravity, materials come together
more slowly, allowing zeolite crystals to form that are both
larger and more orderly." Zeolite crystals produced on Earth
are small, roughly 2 to 8 microns across. "That's about
one-tenth the thickness of a human hair." The ones he grew
on the space shuttle were not only 10 times bigger, but also
better organized internally--a promising start.

"The
next step is the International Space Station," says Sacco.
He and others at CAMMP have built a Zeolite Crystal Growth Furnace,
which was installed on the ISS in early 2002. "Ken Bowersox,
the ISS Expedition 6 commander, has used the furnace to grow
some crystals for us. Ken had to correct some unexpected problems
with the mixing of the crystal growth solution--this shows the
values of humans in space--but after that the experiment went
smoothly."

"Now we need to get those crystals back to Earth where
we can examine them. A few might come down in May," when
the Expedition 6 crew leaves the ISS in an Soyuz capsule. "I'd
really like to see them," says Sacco.

The goal, he says, is not to mass produce zeolite crystals
in space. That's not economical--at least not yet. "We simply
want to find out if it's possible to grow zeolite crystals that
can reach the 7% threshold. If we can do that in space, we'll
figure out how to reproduce the process on the ground."

Throughout his career, Sacco has envisioned a worldwide transition
from fossil to hydrogen fuels. It's a big dream, but it could
happen. "Zeolites may be the key to hydrogen fuel as a leapfrog
technology."

The Science and Technology Directorate at NASA's
Marshall Space Flight Center sponsors the Science@NASA web sites.
The mission of Science@NASA is to help the public understand
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Space
Product Development
-- (NASA/SPD) The goal of NASA's Space Product Development (SPD)
program is to help American businesses explore the potential--and
reap the rewards--of doing
business in space. Doing this helps bring the benefits of
space down to Earth where it can, and does, enrich the everyday
lives of the American public. "Industry investment in space
is high," says Mark Nall, manager of NASA's SPD program
at Marshall Space Flight Center. "We assist companies developing
experiments and help them explore how space research can contribute
to the growth of their businesses."

Zeolites aren't just absorbers, they are also wonderful
filters. They can be found in many water and air purifiers, and
they're widely used by the petroleum industry to extract gasoline
from petroleum. "All of the gasoline in the world is derived
from petroleum using zeolite," says Sacco. Zeolites are
also critical to the manufacture of perfumes and paints. The
production of zeolite for these many applications is a 2 billion
dollar per year industry.

Cool Fuel Cells -- (Science@NASA) Fuel cells promise to be the
environmentally-friendly power source of the future, but some
types run too hot to be practical. NASA-funded research may have
a solution.